UA-33602782-1
Aug27

## CFD keeps Emirates Team New Zealand on course to reclaim the America's Cup

LEAP staff, in particular our team of CFD engineers, have been watching with interest as the 2013 America's Cup unfolds in San Francisco. Despite being the oldest active trophy in international sport, the America's Cup is continually evolving thanks to an often dramatic combination of ...

Aug17

Jun25

## Tips & Tricks: Turbulence Part 2 - Wall Functions and Y+ requirements

Previously we have discussed the importance of an inflation layer mesh and how to implement one easily in ANSYS Meshing.  We also touched upon the concept of mesh y+ values and how we can estimate them during the inflation meshing process.  In other posts, we also discuss the different turbulence models and eddy simulation methods available to ANSYS CFD users.  In today's post, we'll talk in more detail about y+ values apply to the most commonly used turbulence models. From our earlier discussions, we now understand that the placement of the first node in our near-wall inflation mesh is very important.  The y+ value is a non-dimensional distance (based on local cell fluid velocity) from the wall to the first mesh node, as you can see in the image below.  To use a wall function approach for a particular turbulence model with confidence, we need to ensure that our y+ values are within a certain range. Looking at the image above, we need to be careful to ensure that our y+ values are not so large that the first node falls outside the boundary layer region.  If this happens, then the Wall Functions used by our turbulence model may incorrectly calculate the flow properties at this first calculation point which will introduce errors into our pressure drop and velocity results.  The upper range of applicability will vary depending on the flow physics and the extent of the boundary layer profile. For instance, flows with very high Reynolds numbers (typically aircraft, ships, etc) will experience a logarithmic boundary layer that extends to several thousand y+ units, whereas low Reynolds number flows such as turbine blades may have an upper limit as little as 100 y+ units.  In practice, this means that the use of wall functions for these class of flows should be avoided as their use will limit the overall number of mesh nodes that can be sensibly placed within the boundary layer.  In general, it is recommended that you endeavour to place sufficient inflation layer cells within the boundary layer, rather than simply focusing on achieving any particular y+  value. This will be covered in detail in a future post In addition to the concern about having a mesh with y+ values that are too large, you need to be aware that if the y+ value is too low then the first calculation point will be placed in the viscous sublayer (logarithmic) flow region and the Wall Functions will also be outside their validity (below about y+ < 11).  You can imagine that this would become an issue if a mesh intended to be used with wall functions is then refined near the wall.  Fortunately, the use of...